A multicolor photographic material typically comprises a support bearing a cyan dye image-forming unit comprising at least one red-sensitive silver halide emulsion layer having associated therewith at least one cyan dye-forming coupler, a magenta dye image-forming unit comprising at least one green-sensitive silver halide emulsion layer having associated therewith at least one magenta dye-forming coupler, a yellow dye image-forming unit comprising at least one blue-sensitive silver halide emulsion layer having associated therewith at least one yellow dye-forming coupler. One of the challenges of preparing photographic materials is to have each of the red, green, and blue sensitive emulsions absorb light as close as possible to the wavelength of light sensitivity of the human eye in that color range of the spectrum.
The human eye is most sensitive to green light. Thus the green light sensitive layer of photographic materials can have a large impact on perceived color reproduction. This layer is generally sensitive to light within the wavelength region of 500 to 600 nm. In photographic materials, it is common practice to sensitize this layer with a sensitizing dye that has a maximum sensitivity at about 550 nm. However, the human eye has a peak sensitivity at about 540 nm, and still has substantial sensitivity at 500 nm. Efficient sensitizing dyes in the region of 500 to 540 nm would enable more accurate color reproduction for color negative films.
Benzimidazolooxacarbocyanines can provide spectral sensitivity in the region of 520 to 540 nm. However, emulsions containing dyes of this type are known to readily give fog when subjected to heat. They are also known to have poor keeping properties resulting in a loss in sensitivity with time. Also with this dye class, in order to achieve a J-aggregate that absorbs light at a short green wavelength, it is necessary to make the chromophore very unsymmetrical with respect to the charge distribution. This results in a dye with a low extinction coefficient and lowered light absorption per unit dye.
Oxacarbocyanines are another class of dyes that afford efficient J-aggregate sensitization in the green region. Emulsions sensitized with oxacarbocyanines generally do not give fog upon heating and have excellent keeping properties. However, in general, emulsions sensitized with oxacarbocyanines dyes have a maximum sensitivity at 545 nm or greater.
Ikegawa et. al. (U.S. Pat. No. 5,198,332, U.S. Pat. No. 4,970,141, and U.S. Pat. No. 4,889,796) and Nakamura et. al. (U.S. Pat. No. 5,637,448) describe oxacarbocyanine dyes that provide spectral sensitivity below 545 nm. U.S. Pat. No. 5,523,203 describes another class of short green sensitizers. Parton et. al., in U.S. Pat. No. 5,316,904, describe amide-substituted oxacarbocyanine dyes as affording reduced post-process dye stain. However, dyes that give further improvements in spectral sensitivity in the wavelength region of 525 to 535 nm are still needed to improve color reproduction with high sensitivity.
The red sensitivity of the human eye peaks at approximately 590 nm. However, the red wavelength region, 600 to 700 nm, in many photographic products, for example color negative films, has been often sensitized with a dye that has its maximum sensitivity at about 650 nm. A change in the red spectral sensitization from a maximum at 650 nm to a position closer to 600 nm, for example in the 620 to 640 nm region, has several advantages. This could improve the sensitivity of the film color balance to changes in illuminant, especially fluorescent light. Also, some colors that are difficult to reproduce because of high infrared reflectance, would be reproduced more accurately. Thus dyes that have a maximum sensitivity in the short red region, 620 nm-640 nm are desirable.
In many photographic products, for example color negative films, the blue spectral region, 400-500 nm, has been often sensitized with a dye that has its maximum sensitivity at about 470 nm while the eye sensitivity has a peak at approximately 440 nm, and fluorescent lights have a peak emission at 435 nm. A broader blue sensitization envelope could improve the sensitivity of the film color balance to changes in illuminant, especially fluorescent light. This type of spectral envelope can be obtained by combining a dye that has a maximum sensitization at 470 nm with a dye that has a maximum peak at a shorter wavelength. Thus substituents that cause a blue sensitizing dye to aggregate at a shorter wavelength, for example 400-460 nm are desirable.